Many attempts have been made in the past to provide a mechanism for monitoring the condition of a filter in either a fluid or gaseous environment and to detect whether a filter element must be replaced or reconditioned before continuing operation. These devices are, for example, used in hydraulic systems to provide a visual or electrical signal (or a combination of both) when differential pressures across a filter element exceed a set value. Devices of this nature have been fashioned in electrical forms, mechanical forms, or a combination of both. However, problems have arisen with the said devices.
The most cost-efficient indicators to date utilize a combination of mechanical and electrical elements. In these hybrid indicators, a mechanical micro switch is used to provide a signal by opening or closing an electrical circuit. However, the use and reliability of a micro switch is limited when low currents (less than 0.5 amp) are required. Additionally, many applications require hermetically sealed switches, which in turn increase the size, weight, and cost of the indicator.
Thus, there is a need for reliable, small, inexpensive differential pressure indicators that can work with low currents and a broad range of temperatures, especially for applications involving hydraulic systems in aircraft.